PFG-NMR self-diffusion measurements in the single phase channels of a microemulsion system with an anionic–nonionic surfactant mixture

Abstract

The single phase channels of a presently reported microemulsion system were investigated by electrical conductivity and pulsed-field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. The system consists of a mixed anionic–non-ionic surfactant mixture, water and decane. At constant surfactant concentration and temperature, the phase diagram exhibits two single phase microemulsion channels, separated by an anisotropic lamellar channel. The lower microemulsion channel starts from the water side of the phase diagram with a micellar L₁ phase and reaches the middle of the phase diagram with increasing mass fraction of decane in the solvent mixture and increasing mass fraction of lipophilic co-surfactant in the surfactant mixture. The upper microemulsion channel passes from the aqueous side with an L₃ phase to the oil side of the diagram. Conductivity data and self-diffusion coefficients, obtained by PFG-NMR, support the previously made conclusion that the nanostructure in the upper channel undergoes an abrupt transition from a bicontinuous structure to a water-in-oil High Internal Phase Microemulsion (HIPME) with already less than 10% of oil in the solvent mixture, while the structures in the lower microemulsion channel are oil-in-water droplets. The HIPME structure is a feature of the surfactant mixture and probably formed due to a high interfacial tension between the aqueous diluted surfactant phase and the oil. By the addition of salt, the HIPME structures are obviously disturbed, resulting in an increased conductivity and self-diffusion rate for the water fraction.